1. Field of the Invention
The present invention relates to an overvoltage-protective device for a power system that uses a DC/DC converter. More particularly, it relates to an overvoltage-protective device for power system wherein direct current (DC) power converted from alternate current (AC) power is used as input power of a DC/DC converter.
2. Description of Related Art
For power system used for various electric appliances of recent years such as desk-top type personal computers, computer game appliances and the like, there has been proposed an overvoltage-protective device that sets input power of a DC/DC converter overload-state and melts a fuse disposed on a current path of the input power with heat so as to stop supplying input power when direct current (DC) power is in an overvoltage-state. This is intended to secure safety of appliances.
FIG. 5 shows a circuit diagram of an overvoltage-protective device for a power system 100 directed to prior art. For various electric appliances of recent years such as desk-top type personal computers, computer game appliances and the like, for example, commercial alternate current (AC) power (for example, 100 V, not shown) is inputted to an AC/DC converter 110 as input power, converted into direct current (DC) power VIN, outputted therefrom as output power, and finally inputted to a DC/DC converter 120 through a fuse 130 as output power.
The DC/DC converter 120 is a drop-voltage type converter constituted by circuits of synchronous rectifying system. That is, an MOS transistor Tr1 as a main-side switching element and an MOS transistor Tr2 as a synchronous-side switching element are switched alternately, thereby to control output power VO to a predetermined voltage VO.
A drain terminal and a source terminal of the MOS transistor Tr1 are connected to output power VIN for the AC/DC converter 110 and a drain terminal of the MOS transistor Tr2, respectively, whereby a node VS is constituted. Furthermore, a source terminal of the MOS transistor Tr2 is connected to a ground potential. The node VS is connected to the output power VO through a coil L1 and as well as connected to a cathode terminal of a diode D1 whose anode terminal is connected to a ground potential so as to supply the output power VO flyback current derived from back electromotive force of the coil L1 from the ground potential. A capacitor C1 for voltage smoothing is connected to the output power VO.
From a control circuit 31, output signals OUT1 and OUT2 are outputted to gate terminals of the MOS transistors Tr1 and Tr2, respectively. In normal use, switching of the output signals OUT1 and OUT2 are made alternately based on terminal voltage of the output power VO detected by the control circuit 31, thereby to control output voltage to a predetermined voltage value. Furthermore, the output voltage VO is detected by an overvoltage detecting circuit 132. Detecting overvoltage state wherein output voltage VO exceeds the predetermined voltage value, the overvoltage detecting circuit 132 outputs an overvoltage detection signal to the control circuit 31. Upon receipt of the overvoltage detection signal, so as to lower voltage value of the output power VO, the control circuit 31 makes a control signal OUT1 low level to set the MOS transistor Tr1 OFF-state whereas makes a control signal OUT2 high level to set the MOS transistor Tr2 ON-state. Thereby, a current path coming from an input power VIN for the output power VO is cut off and at the same time, the output voltage VO is lowered by connecting the output power VO to a ground potential via the coil L1. There is thus constituted an overvoltage-protective device that is to avoid applying overvoltage to electric devices such as CPU.
Let us take a case that a line between the drain terminal and the source terminal of the MOS transistor Tr1 is short-circuited due to failure of the MOS transistor Tr1 or the like, which leads the output power VO to an overvoltage-state. In this case, a control signal OUT1 cannot control the MOS transistor Tr1. As a result, the input power VIN and the output power VO are directly connected through the coil L1, whereby the overvoltage detecting circuit 132 detects overvoltage-state at the control circuit 31. Then, a control signal OUT2 in high level is outputted to keep the MOS transistor Tr2 ON-state. That is, there is formed a low-resistance current path running through a ground potential from the input power VIN via the MOS transistor Tr1 destroyed by short-circuiting to the MOS transistor Tr2 that is conductive, and large amount of current flows in the current path. The large amount of current melts the fuse 130 on the current path with heat, whereby the current path is cut off and the DC/DC converter is separated from the output power VIM for the AC/DC converter to stop supplying input power VIM from there. The system of the above prevents electric devices such as CPU from being destroyed.
Furthermore, an under voltage lockout circuit 33 detects the input power VIN for the DC/DC converter. In case that voltage of the input power VIN lowers the predetermined voltage value, the under voltage lockout circuit 33 sets control signals OUT1 and OUT2 low-level so as to make the MOS transistor Tr1 and Tr2 OFF-state, thereby to prevent the DC/DC converter from operating in error during low input voltage period. In case the fuse 130 is melted with heat, supply of input power VIN to the DC/DC converter is stopped. Accordingly, the under voltage lockout circuit 33 sets the MOS transistors Tr1 and Tr2 OFF-state to let the DC/DC converter stop operating.
However, in the overvoltage-protective device for the power system 100 directed to FIG. 5, the fuse 130 must be inserted on the current path running between the AC/DC converter 110 and the DC/DC converter 120 as an overvoltage-protective device for protecting the output power VO from being destructed due to short-circuiting at the drain terminal and the source terminal of the MOS transistor Tr1. As a result, the number of components increases and so does cost for components whereby, price-down of power system cannot be achieved. Furthermore, a mounting region for the fuse 130 must be taken, and necessity to replace a fuse 130 with new one, in case the fuse 130 is destroyed due to overvoltage-protective operation, must be assumed. Thus, mounting condition is significantly limited. Still further, protecting operation against overvoltage accompanies heat to melt the fuse 130 and the heat does harm to the mounting substrate.
The present invention is intended to solve the foregoing prior art deficiency. Its prime object is to provide an overvoltage-protective device capable of protecting a power system from overvoltage not destructively without using a fuse.
In order to achieve the above objective, the overvoltage-protective device for a power system based on one aspect of this invention including an AC/DC converter and a DC/DC converter to which output power of the AC/DC converter is inputted as its input power comprises: an overvoltage detecting circuit for detecting overvoltage state of output power of the DC/DC converter; an alarm circuit for outputting an alarm signal as a detection result obtained by the overvoltage detecting circuit; and a change circuit for changing output-power characteristics of the AC/DC converter based on an alarm signal.
In the inventive overvoltage-protective device for a power system, when the overvoltage detecting circuit detects that the output power of the DC/DC converter turns into overvoltage-state, the alarm circuit outputs an alarm signal. Then, the change circuit changes output-power characteristics of the AC/DC converter based on the alarm signal.
By changing output-power characteristics of the AC/DC converter based on an alarm signal that indicates the overvoltage detecting circuit has detected an overvoltage-state, output-power capability of the AC/DC converter can be limited to the extent that the output power for the DC/DC converter cannot keep overvoltage-state. As a result, there can be avoided overvoltage-state at the output power for the DC/DC converter. Accordingly, it is not necessary to insert a fuse on a current path that connects an output of the AC/DC converter and an input of the DC/DC converter. Thereby, there can be obtained merits as follows: (1) costs for components are reduced; (2) mounting regions for a fuse and the like are not required; and (3) limitations of mountings (replacement of fuses, damages to components caused by heat generated while melting a fuse, and the like) are cleared. That is, an overvoltage-protective device for power system can be realized for sure with low cost and simple structure.
The above and further objects and novel features of the invention will more fully appear from following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustration only and not intended as a definition of the limits of the invention.